Lightning Protection of Antennas ?
Greg Brown
Can anybody help with some guidelines to prevent future shock?
We have a antenna mounted on a mast attached to a chimney with a coax feed
which then goes to different apartments via splitters. It's a house on a
hill,
the highest thing around.
1. What about a lightning rod on an adjacent chimney or maybe the same
chimney?
Should it be higher than the antenna?
2. Should the antenna mast be grounded via 8 ga wire run into an 8 foot
grounding spike?
3. The coax I'm thinking should pass thru a separate (from the mast)
grounding block that goes via 8 gauge wire to a water pipe, a different
grounding point.
4. How do lightning rods work? don't they ionize the air around the tip, so
that
if lightning wants a path to ground it will probably follow the ions to the
tip
of the rod and thence to ground?
5. I suppose one could also add a surge protector to the incoming electric
power feed as well.
Any thoughts would be appreciated, before the next storm!
--
Internet: gbr...@va.med.umich.edu
Gary Coffman
>Help, Lightning blew up everybodies electronics in my apt house.
>
>Can anybody help with some guidelines to prevent future shock?
>We have a antenna mounted on a mast attached to a chimney with a coax feed
>which then goes to different apartments via splitters. It's a house on a
>hill,
>the highest thing around.
>
>1. What about a lightning rod on an adjacent chimney or maybe the same
>chimney?
>Should it be higher than the antenna?
Lightning rods won't hurt, *if installed properly*, but they're no
guarantee that the lightning strike won't induce damaging currents
on other nearby conductors. The lightning rod should be the highest
thing around, but note that this almost assures that it *will* be hit
in storms, so you'd better know what you're doing when you install it.
>2. Should the antenna mast be grounded via 8 ga wire run into an 8 foot
>grounding spike?
Absolutely, and at minimum. That's the minimum NEC spec. Ideally you'll
use a better downlead and a better ground than that. Five inch copper
strap, run with *no sharp bends* straight to the ground terminal is
advised. Also a single ground rod is often not a good enough ground.
A better ground field will use several 8 foot rods, spaced no closer
to each other than 8 feet, all connected together with heavy strap
in a *star* configuration. You should also tie all other grounded
items, like water pipes, power company ground, telco ground, etc,
to this central star. That central star connection should be
the *only* place you make ground connections to protected structures
and equipment.
>3. The coax I'm thinking should pass thru a separate (from the mast)
>grounding block that goes via 8 gauge wire to a water pipe, a different
>grounding point.
No, No, NO! All grounds must be common to avoid ground loops that will
allow damaging currents to flow through your equipment. *All* ground
connections must be to the single point ground you establish with the
star configuration mentioned above. Do use a good gas discharge suppressor
in the antenna downlead, but make sure it's grounded to the central
common point ground via a low inductance conductor. The suppressor
should be installed at the point where the downlead enters the
structure.
>4. How do lightning rods work? don't they ionize the air around the tip, so
>that
>if lightning wants a path to ground it will probably follow the ions to the
>tip
>of the rod and thence to ground?
It's more complex than that in reality, but that's the general idea.
Lightning, like any current, will preferentially take the lowest impedance
path to complete the circuit. Ideally, the lightning rod supplies that
path. In practice there are other considerations, primarily induction
issues, that assure there *will* be current sharing with other conductors.
That's why it's critical that all exposed conductors be tied to a *common*
ground point. That equalizes their potentials and prevents circulating
currents.
Streamers, corona or ion discharges, always form prior to any lightning
strike. Sharp points can promote earlier streamer formation by requiring
a lower ionization potential. So a sharp pointed lightning rod is typically
the first good grounded conductor to begin the streaming that leads to
a strike. That makes it the preferred path for the strike. But, if the
rod doesn't have the lowest impedance path to ground (inductance is
particularly critical), then the main bolt may jump to a nearby conductor
that *does* have a low impedance path to ground. That's why it's so
important to have a straight path with no sharp bends from the rod to
ground. Wide strap conductors have lower inductance than round wires,
so use of wide strap conductors is preferred. (Copper flashing available
from roofing supply houses in long rolls is a good conductor to use for
low inductance grounds.)
>5. I suppose one could also add a surge protector to the incoming electric
>power feed as well.
Yes. *All* wires entering or leaving the premises should be fitted with
proper surge suppressors. That includes power, telco, CATV, antenna leads,
rotator control wires, etc. All of the suppressors should be connected
*directly* back to the common star ground connection to assure a single
point ground and eliminate harmful ground loop currents.
Lightning rods can sometimes be helpful, but the more important things
you can do for protection are the single point ground system and the
low inductance ground connections to it from all equipment and surge
suppressors. If you can tie all the conductors in the vicinity to a
good common ground point, you can prevent the *differential* potentials
between conductors that do the majority of damage in any lightning
strike. Remember lightning is a spark discharge, and like a spark
transmitter, the energy of the bolt is *RF*, so use good RF grounding
practices to deal with it.
Gary
--
Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | we break it. | uunet!rsiatl!ke4zv!gary
534 Shannon Way | Guaranteed! | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244 | |